Even though plastic contact lenses have been available for many years, plastic photochromic contact lenses remain relatively rare. A photochromic hard contact lens is disclosed in Japanese patent publication no. JP 4-208919 (1992), and JP 63-306418 (1988).
It is desirable to have a photochromic material which can be used in the production of a contact lens whose transmittance varies as a function of light. Outside of their photochromic properties (i.e., colorability, rapid darkening and lightening kinetics, acceptable durability, etc.), such lenses are generally made by the use of appropriate mixtures of photochromic compounds. Even though the polymer matrix used for such lenses are thermally crosslinked, they tend to have a low glass transition point, generally lower than that of CR39.RTM., a reference ophthalmic resin consisting of diethylene glycol bis(allyl carbonate) available from PPG Industries, in order to exhibit rapid photochromic kinetics. Moreover, these polymers generally have a relatively low refractive index (below 1.54).
The majority of these thermally crosslinked matrixes are obtained by radical polymerization (i.e., polymerization which most often can only be carried out provided that one uses initiators of the organic peroxide type.) The use of organic peroxides makes it practically impossible to incorporate photochromic molecules in the mixture of monomers before polymerization, the peroxides having the effect either of destroying any photochromic effect or of giving the product an unacceptable permanent intense coloration.
Therefore, there continues to be a need for photochromic organic contact lenses having improved photochromic properties and which are relatively easy to manufacture.